Method for determining the coating weight of tin on tin plates



July 2, 1940.

Filed Oct. 28, 1937 J. S. BUSER METHOD FOR DETERMINING THE COATINGWEIGHT 0F TIN ON TIN PLATES MOW/Q43 s Patented July 2, 1940 UNITEDSTATES PATENT OFFICE METHOD FOR DETERMINING THE COATING WEIGHT OF TIN ONTIN PLATES Joseph S. Buser, Toronto, Ohio Application October 28, 1937,Serial No. 171,551

2 Claims.

purposes.

By the present method the tin coating is removed from the sampleelectrolytically in the presence of a known quantity of an oxidizingagent. The excess of the oxidizing agent present in the electrolyte isthen determined and the Weight or thickness of the tin coating is thencalculated from the data obtained.

While the method described herein may be applied to many types of metalcoatings, it will be described herein specifically for the determinationof the coatings of tin on tin plate. The reagents employed in thepresent process may also be varied as is well understood by personsskilled in the art.

In the present method a sample of tin plate of standard dimensions issuspended in the electrolyte of an electrolytic cell, the cell beingconnected so that the sample constitutes the anode of, the cell, carbon,preferably in cylindrical form, being employed for the cathode. Aconstant current is permitted to flow through the cell for apredetermined period of time, the current and time being so determinedthat substantially all of the tin will be removed from the tin plate. Itis important that in the conduct of this process the current should notbe permitted to pass through the cell after substantially all of the tinhas been removed, for the reason that such continued flow of currentthrough the cell may produce reactions between the ingredients in theelectrolyte which may interfere with the results of the determination.

The electrolyte preferably contains hydrochlo ric acid, potassiumiodate, and potassium iodide, all in aqueous solution. The passage ofelectricity through this cell results in the dissolution of the tin,which in the absence of the oxidizing agent present would be convertedinto the salt of the lowest valence and result in the formation ofstannous chloride. In the present case, however, the oxidizing agentpresent causes the oxidation of the bivalent tin to its tetravalentform, resulting in the formation of stannic chloride. The oxidation inthe present case is the result of the action of the iodine which isliberatedby the reaction between potassium iodate and potassium iodidein acid solution.

In the conduct of the present process an excess of the oxidizing agentor of the reagents which produce the oxidizing agent should be bad. Whenthe detinning is completed, the flow of current through the cell isinterrupted, and the excess of iodine or potassium iodate stillremaining is determined by titrating the electrolyte with sodiumthiosulphate in the presence of starch as an indicator.

Knowing the amount of potassium iodate originally introduced into thecell and knowing the amount of potassium iodate which remained in 16 thecell after the detinning was completed, the amount of potassium iodateconsumed in reaction with the dissolved tin is a measure of the tinremoved from the sample, and thus is a measure of the thickness or theweight of the tin coating.

It will now be understood that while in the preferred form of thepresent invention the tin is removed electrolytically in the presence ofan excessive amount of an oxidizing agent,

instead of an oxidizing agent, other agents may 35 be employed, providedthey be such as will react quantitatively with the dissolved tin. In thepreferred method as described herein the conduct of the determination isgreatly facilitated for the reason that the electrolytic solution can betitrated immediately after the completion of the detinning step withoutany intermediate treatment of the solution.

, The chemical reactions upon which the present process is based are asfollows:

In the aqueous solution of acid, iodate, and iodide, the followingreaction takes place:.

The solution employed should contain a known quantity of potassiumiodate and an excess of potassium iodide and acid.

Instead of the potassium iodate, other agents for liberating iodine maybe employed, such as potassium biiodate, potassium permanganate, orpotassium dichromate.

When the tin goes into solution by electrolysis SnClz is originallyformed and then is actedupon by the iodine as follows:

Equations (1) and (2) thus show that the amount of iodine liberated isdetermined by the amount of potassium iodate or other such iodineliberating oxidizing agent employed. The free iodine oxidizes the tin.The excess iodine is determined by titration with sodium thiosulphate inaccordance with the following equation:

Thus, the amount of tin which was placed in solution by electrolysis isdetermined by the aid of the above equations.

The drawing accompanying the present application illustrates anapparatus that may be employed in the conduct of the present process.

In the drawing:

Fig. 1 is a plan view of the electrolytic cell employed and alsocontains a diagrammatic showing of the electrical wiring systememployed;

Fig. 2 is a front partly sectional view of the apparatus; and I Fig. 3is a side partly sectional view of the apparatus.

Referring to the drawing, 10 designates a glass container, the glasscontainer being employed for the reason that the titration of the excesspotassium iodate may, if desired, be conducted in this container, takingcare, however, that the solution adhering to the anode andcathode shouldbe washed into the container. The carbon cathode ll, preferably in theform of a hollow cylinder, is placed in the container and is connectedto the source of direct current by means of the cathode connector l2 andwire l3. The sample of tin plate It is suspended within the cathode bymeans of the anode clamp l5 and the anode connector I6 which rests ontop of the container l0. Interposed between the anode and the positiveside of the source of current is the rheostat H, the ammeter l8, and theautomatic time switch l9.

In the conduct of the present process the electrolyte consisting of aknown amount of standard potassium iodate solution together with anexcess of potassium iodide and of hydrochloric acid, and enoughdistilled water to cover the anode, is placed in the glass container. Aconstant electric current is passed through the cell for a definiteperiod of time, the automatic time switch controlling this timeinterval.

For removing the coating from a sample two inches square and having acoating weight of 1 lbs. per base box, a current of 1 ampere for aperiod of three minutes is employed. After the lapse of three minutes,at which time the automatic switch will open the circuit, the sample isremoved from the electrolyte and washed,'the wash water being added tothe electrolyte. Similarly, the cathode is removed and washed, the washwater being added to theelectrolyte. The excess potassium iodate or itsequivalent, the excess iodine liberated by the potassium iodate, is nowdetermined by titration with standard sodium thiosulphate solution,starch being used as an indicator.

In the calculation of the weight of the tin coat-' ing allowance is madefor a certain constant which is characteristic of the cell as operated.This cell constant is obtained by comparing the results obtained by achemical analysis of a specimen of tin plate with the results given bythe present process on a specimen of the same tin plate.

It is found that in the deposition of tin during the manufacture of tinplate some iron-tin alloy is formed which does not yield up its tincontent by the present process. There is also a small amount of tindeposited in metallic form on the cathode. The electrolysis also hassome effect upon the chemical reagents. It is found that in the presentprocess, where the current is constant and the time is constant, thesefactors stay constant. Together these are here termed a cell constantwhich is determined originally by comparing the results obtained by achemical analysis of a specimen of tin plate with the results given bythe present process of a specimen of the same tin plate. In carrying outmy present process I employ for my cell a glass beaker within which aremounted two carbon electrodes three inches by one inch by inch withfaces parallel and 1% inches apart. The specimen two inches square isdisposed between the electrodes. The electrolyte consists of 200 cubiccentimeters of hydrochloric acid (1 part concentrated HCl and 20 partswater) and 15 cubic centimeters of standard potassium iodate solutionequivalent to exactly .007 gram of tin per cubic centimeter. Thepotassium iodate solution contains dissolved therein .04 gram ofpotassium iodide per cubic centimeter. A current of 1 ampere is passedthrough the cell for a period of three minutes, sufficient to remove thetin from a specimen having a coating weight of 1 lbs. per base box,typical of commercial tin. The amount of tin is then determined by theamount of potassium iodate used from the electrolyte. It has been foundby a comparison of over a thousand tests made simultaneously by theelectrolytic method using the cell described above and a standardchemical method, that the electrolytic method gives results which showthe tin content to be .007 gram higher than that shown by the standardchemical method. Therefore the cell constant for the above describedcell is minus .007

gram of tin, which constant remains the same under the above mentionedconditions of time, current, concentration of electrolyte and cellconstruction.

While the process is described herein as specifically applied to tincoatings, the process is also applicable to other metal coatings, aschromium, nickel, zinc. and the like, provided that the reagentsemployed are such that they will react quantitatively with the dissolvedmetal.

In lieu of the oxidizing agents that have been set forth in earlierportions of the present specification, other reagents may be employedwhich have the quality of reacting quantitatively with the dissolved tinor other metal. Among such other reagents may be soluble sulphides,hydrogen sulphide in aqueous solution, and oxalates.

Other iodine solutions than the type described hereinabove may beemployed, such as iodine in excess potassium iodide; chlorates, andbromates may be used for the liberation of iodine.

Instead of mineral acid in the electrolyte, an organic acid such asacetic acid may be employed.

Having thus described my invention, I claim:

1. The method of rapidly and accurately determining the weight of tincoatings on a plurality of specimens of tin plate or the like, whichcomprises first determining the correction factor of an electrolyticcell in which the electrolyte contains hydrochloric acid, potassiumiodide and an iodine liberating agent of the group consisting ofpotassium iodate, potassium permanganate, and potassium dichromate andhaving a tin plate anode, which factor is derived from a comparison ofthe results obtained by chemical analysis with the re- I sults obtainedby passing a predetermined constant electric. current through said cellfor a predetermined time, and thereafter subjecting each specimen toelectrolysis for the same period of '15 time and under the same constantcurrent conditlons in the same cell thereby dissolving substantially allthe tin from the specimen, determining the excess of iodine andcorrecting the results thus obtained by said correction factor.

2. The method of rapidly and accurately determining the weight of tincoatings on a plurality of specimens of tin plate or the like, whichcomprises first determining the correction factor of an electrolyticcell in which the electrolyte contains hydrochloric acid, potassiumiodide and an iodine liberating agent of the group consisting ofpotassium iodate, potassium permanganate, and potassium dichromate andhaving a tin plate anode, which factor is derived from a comparison ofthe results obtained by chemical analysis with the results obtained bypassing a predetermined constant electric current through said cell fora. predetermined time, and thereafter subjecting each specimen toelectrolysis for the same period of time and under the same constantcurrent conditions in the same cell thereby dissolving substantially allthe tin from the specimen,. determining the excess of iodine bytitration with sodium thiosulphate and correcting the results thusobtained by said correction factor.

JOSEPH S. BUSER.

